A polyethylene porous film is used as a separator of a lithium ion battery. In order to make the voltage of the lithium ion battery higher, it is effective to increase the pore volume of a separator so that lithium ions can further smoothly move between electrodes. Thus, although a method of increasing the pore diameter of the separator has been employed, when the pore diameter is 1 μm or more, lithium ions in an organic electrolyte crystallize and deposite in a dendrite (arboroid) form as metallic lithium, whereby energization occurs, so that there occurs a problem that short circuiting of a battery occurs. Accordingly, in order to improve the performance of a lithium ion battery, it is required to realize such conflicting pore characteristics that a pore volume and a specific surface area increased while suppressing a pore diameter of a separator to an nm order.
Currently, pores of a polyethylene porous film used as a separator of a lithium ion battery are formed by extracting and removing an organic solvent previously impregnated in a polyethylene film as a raw material (Japanese Patent Application Laid-Open (JP-A) No. 2004-182763). In addition, a method of drawing a polyethylene film impregnated with an organic solvent, then extracting and removing a solvent, and thereby forming a porous film has been proposed (JP-A No. 2003-103626).
In those conventional processes for producing a polyethylene porous film, a phase separation structure including an organic solvent and a polyethylene component is a precursor of a porous structure, and therefore, when pores of an nm order are formed by extraction and removal of the solvent, the same or a larger amount of the solvent as/than the amount of the polyethylene component contained in a polyethylene film as a raw material is required to be impregnated. Accordingly, the production of the polyethylene porous film in which the pores are formed by removal of a large amount of the organic solvent is complex, and environmental load with volatilization of the organic solvent becomes problematic.
Meanwhile, a process for adding an inorganic fine powder with such an organic solvent, dissolving and removing them, and thereby producing a polyethylene porous film is disclosed (JP-A No. 2010-7053). However, a process for dissolving and removing the inorganic fine powder is also complex, and there is a restriction that an obtained pore diameter in principle cannot be made smaller than the size of the inorganic fine powder used in the formation of pores.
Meanwhile, the present inventors filed an application (WO 2010/101214) in which a process for producing a polyethylene porous film is disclosed, the process contains biaxially drawing an ultrahigh molecular weight polyethylene film in a molten state (at a temperature greater than or equal to the melting point of the film), then subjecting the film to a shrinkage treatment so as to express a lamellar structure having a uniform thickness, and redrawing the film in a solid-phase state (at a temperature less than or equal to the melting point of the film) to form pores in the film, whereby the polyethylene porous film having the pores of an nm order is obtained. According to the production process, since the pore formation by extraction and removal of an impregnated organic solvent and the pore formation by dissolution and removal of an added inorganic substance are not accompanied, producing a polyethylene porous film with less environmental load is possible. However, the shrinkage treatment after melt-drawing is required in this production process, and thus this production process has a room for improvement in terms of efficient production of the polyethylene porous film achieved by fewer steps.
In the above lithium ion battery, a method of increasing the number of stacked electrodes has been currently employed for realizing high output. In a flat-plate shaped lithium ion battery used in a cell phone and so on, a separator film is wound with an electrode around a core at high tension, and this wound body is formed into a flat plate shape, whereby the lithium ion battery is assembled. In this case, a separator is in a state of being apt to be ruptured by the tension in winding an electrode layer and bending in the formation into a flat plate shape. Accordingly, it is desirable to use a high strength polyethylene porous film as a separator for a lithium ion battery.
In addition to the above effects, the increase in the strength of the separator film is effective for reducing the thickness of a cell stack and realizing high integration. When the electrode layers are wound and stacked at the same tension, a separator film having a higher film strength has an advantage that it is not raptured even if the thickness of the separator film is further reduced. The reduction of the thickness of the separator film can increase the number of stacked electrode layers, and thus the realization of high output can be expected.
As means for increasing the strength of a nonporous polyethylene film, there has been proposed means for adding ultrahigh molecular weight polyethylene to normal molecular weight polyethylene (100,000 molecules or less) and drawing the polyethylene film in JP-A No. 2010-167640. It also describes that a film excellent in tensile strength and impact resistance is obtained by this means. Although a drawn polyethylene film described in JP-A No. 2010-167640 has excellent tensile strength and impact resistance and is useful as a protective member for a human body, the content of ultrahigh molecular weight polyethylene is so small as not more than 40% by mass, and it is not studied that the polyethylene film is made porous.